If you add up the Watts used by your hardware and the sum is larger than the watts provided by your UPS, that's usually not good.
–
Oliver SalzburgAug 5 '12 at 12:24

1

Technically it depends on the specific UPS design but I would not generally recommended it. Though you can work near it's maximum and reduce consumption when triggered to stretch the up time.
–
amotzgAug 5 '12 at 12:37

1

Expanding on UPS design: There are a kind of passive UPSes. They detect power failure and quickly switch to batteries supplied power. And active UPSes which always provide power from batteries while continuously charging.
–
HennesAug 5 '12 at 13:03

1

On a different note: Can it support extra hardware today (with fresh batteries) or can it support it over a year or two (with batteries near replacement again). Plan for the last case.
–
HennesAug 5 '12 at 13:06

2 Answers
2

If you have 8.5 Ah and draw 8.5 amps, you can do it for one hour. Conversely you can draw 5.8 Amps for 32% longer or approximately 81 minutes. You should only try to draw 80% of your max rating. Batteries get HOT under 100% load.

I would be nervous about operating so close to the stated maximum wattage of the UPS's. You should upgrade as soon as possible to the next available size (wattage).

The formula for wattage is very simple. It is Volts (electrical pressure) times Amps (electrical current). So a 120 Volt 5 Amp (maximum current draw) device would need a 600 Watt power supply.

What would all of this information provide you? Figure out the total in kilowatts, and then multiply that by the number of hours it is on per day (daily kilowatt hours). Then multiply that by the cost per kilowatt, and you now know what this rack of equipment costs you in electricity every day.

First thing to do is to ignore the nameplate and actually measure the current draw. Get an electrician to do that if you don't have the gear for it. Make measurements at the heaviest load your servers normally experience. It's surprising how different (and generally lower) real-life figures are compared to the server specs.

Next ensure your calculated figure is no more that half of the UPS rating. While you could theoretically draw the full Ah rating of the battery for an hour, drawing anything more than half that figure will result in a drastic reduction of the battery life. Ideally even stay below one quarter of the rating.

Of course your UPS should also state the maximum current it can supply, so be sure to factor that in as well.

If you're using an on-line UPS also remember that the UPS has to be able to supply your expected startup load. Powering up a rack of servers (spinning up the hard drives and fans) often draws even more wattage than "peak load" for a machine that's running. You need to plan for this by either allowing extra UPS capacity or having a specific power-up sequence to avoid overloading the UPS.
–
voretaq7♦Aug 10 '12 at 16:42